Acrylic structural adhesives are extensively used for providing structural strength-imparting bonds to joined metal and/or polymer materials. Acrylic structural adhesives are useful for bonding of metal parts in place of welding or mechanical fastening techniques. The structural requirements include high bond strength and good failure mode. A typical method to measure bond strength is the T-peel test. One prevalent use for acrylic structural adhesives is in forming hem flanges in automotive body panels and doors. Exemplary conventional acrylic structural adhesives and methods for using acrylic structural adhesives are disclosed in the following U.S. Patents:    U.S. Pat. No. 6,180,199 entitled Beaded Adhesive And Hem Flanged Part Made Therefrom;    U.S. Pat. No. 6,074,506 entitled Method Of Bonding Using Non-Compressible Beads;    U.S. Pat. No. 5,932,638 entitled Free Radical Polymerizable Compositions Including Para-HalogenatedAniline Derivatives;    U.S. Pat. No. 5,783,298 entitled Adhesive Mixture With Non-Compressible Beads Therein;    U.S. Pat. No. 5,710,235 entitled Olefinic And Urethane-Terminated Ester Polyalkadiene;    U.S. Pat. No. 5,641,834 entitled Modified Polyalkadiene-Containing Compositions; and    U.S. Pat. No. 5,632,413 entitled Adhesive Bonding Apparatus And Method Using Non-Compressible Beads.
Conventional acrylic structural adhesives typically comprise a mixture of one or more olefinic reactive monomers such as methyl methacrylate and methacrylic acid, toughener(s) and redox initiator system. The toughener(s), which may or may not be reactive, or polymerizable with the reactive monomers. Reactive polymers such as unsaturated polyesters, acrylourethane prepolymers may used to grafting onto or crosslink the initiated monomers during polymerization. In addition, fully formulated acrylic structural adhesives typically contain other additives for improving adhesion to substrate materials, environmental resistance, impact strength, flexibility, heat resistance, and the like. Epoxy resins impart improved heat resistance.
U.S. Pat. No. 5,932,638 discloses a structural adhesive which utilizes a redox catalyst system that includes certain para-halogenated aniline reducing agents. U.S. Pat. No. 4,769,419 (“the '419 patent”), incorporated herein by reference, discloses structural adhesive compositions for metal-metal bonding applications. The adhesives include as tougheners olefinic terminated liquid rubbers which are reacted with monoisocyanate compounds. These tougheners are urethane modified olefinic-terminated liquid elastomer produced from a carboxylic acid-terminated alkadiene or, alternatively, as isocyanate-capped methacrylate-terminated polyalkadiene produced from a carboxylic acid-terminated polyalkadiene. The '419 patent further describes acrylic adhesives with olefinic monomer, olefinic urethane reaction product of an isocyanate-functional prepolymer and a hydroxy-functional monomer, phosphorus-containing compounds, an oxidizing agent and a free radical source.
U.S. Pat. Nos. 5,641,834 and 5,710,235, both incorporated herein by reference, disclose adhesives that include as tougheners an olefinic-terminated polyalkadiene that includes carboxy ester linking groups and at least one nascent secondary hydroxyl group that is capped with a monoisocyanate. These tougheners represented by the formulae for polymer A or polymer B in claim 1 are referred to herein as “olefinic-terminated liquid elastomer produced from a hydroxyl-terminated polyalkadiene” or, alternatively, as “isocyanate-capped methacrylate-terminated polyalkadiene produced from a hydroxyl-terminated polyalkadiene”. The composition also includes a free radical-polymerizable monomer such as an olefinic monomer and, optionally, a second polymeric material. In a preferred embodiment the composition is an adhesive that also includes a phosphorus-containing compound and an ambient temperature-active redox catalyst.
U.S. Pat. No. 5,728,759 (“the '759 patent”) discloses non-structural waterbased pressure sensitive adhesives and methods of preparation including an adhesive entity, a tackifier resin, a plasticizer, stabilizer, curing entity and optional additives. Objects of the '759 patent include providing masking tapes that do not freeze to an automobile body or window on exposure of the tapes to relatively high temperatures while in contact with such surfaces. Elastomeric block copolymers usable in the adhesive entity include block copolymers “EUROPRENE.™. Sol T 193A” from Enichem AMERICAS.
U.S. Pat. No. 5,500,293 is directed to a non-structural adhesive composition suitable for use in an insulating tape having improved plasticizer resistance, including from about 13% to about 42% of a polyisoprene homopolymer, from about 13% to about 42% of a styrene-isoprene-styrene copolymer, and from about 25% to about 55% of an aliphatic tackifying agent. The styrene-isoprene-styrene copolymers disclosed include EUROPRENE.™.
Huang, Righettini and Dennis disclose in U.S. Pat. No. 6,225,408 as the curable adhesive portion a mixture of 10–90% by weight of at least one free radical-polymerizable monomer, optional adhesion promoter, a primary toughener with a weight average molecular weight (Mw) less than about 18,000 and an auxiliary toughener with a Mw greater than about 18,000.
Peel strength and failure mode of the current acrylic structural adhesives leaves room for improvement. In investigating different principal methacrylic ester monomers from among the myriad methacrylates, many are disqualified due to objectionable odor or lower T-peel strength on electrogalvanized steel. A state of the art epoxy modified acrylic based upon tetrahydrofurfuryl methacrylate yields 39–41 pounds per linear inch T-Peel on electrogalvanized steel. Substitution of certain alcohols in the ester moiety of the methacrylate esterification lead to surprising improvements in T-peel strength, and these monomers providing improved T-Peel do not exhibit objectionable odor. In another aspect, it was discovered that conventional acrylic structural adhesives containing an epoxy curative in a particular mix ratio limiting the epoxy resin content exhibited significantly improved T-peel strength.